According to this article, Google Android will surpass Microsoft Windows in terms of number of units shipped in 2016. From the article:
More than 90 per cent of the world's PCs still run Windows - but the whole technology landscape is changing around Microsoft's operating system.
By 2016, Microsoft's leading market share of 35.9% [in 2011] will slip to 25.1%. Devices running Google's Android will climb from 29.4% to 31.1%."
Smartphones will show the greatest growth, growing from 916 million shipped last year, to a predicted 1.84 billion units in 2016.